Thermodynamic assessment of the Nb–Ge system

The Nb–Ge binary system has been thermodynamically assessed using the CALPHAD (Calculation of Phase Diagrams) approach on the basis of the experimental data of both the phase equilibria and the thermochemical properties. The reasonable models were constructed for all the phases of the system. The liquid and the terminal solid solution phases, Bcc-(Nb) and Diamond-(Ge), were described as the substitutional solutions with Redlich–Kister polynomials for the expressions of the excess Gibbs free energies. The intermediate phases (Nb3Ge), (Nb5Ge3), (Nb3Ge2) and (NbGe2) with homogeneity ranges were treated as the sublattice models Nb0.75(Ge,Nb,Va)0.25, Nb0.5(Nb,Ge)0.125(Ge,Va)0.375, (Nb,Ge)0.222(Nb,Ge)0.333Nb0.333(Ge,Va)0.111 and (Nb,Ge)0.333(Nb,Ge)0.667 respectively based on their structure features of atom arrangements. A set of self-consistent thermodynamic parameters for the Nb–Ge system was obtained. Using the present thermodynamic data, the calculation results can reproduce the experimental data well.

Research highlights▶ For the phases Nb3Ge2 and Nb3Ge, the reasonable sublattice models were constructed. ▶ A set of thermodynamic parameters for the Nb-Ge system was obtained. ▶ The optimized result can interpreter the vaporization of Ge during preparation.